1. Field of the Invention
This invention generally relates to electrostatic discharge-protective materials, and more particularly to such materials for supporting static-sensitive electronic components having electrical leads.
2. Description of the Prior-Art
Electrostatic discharge (ESD) is widely recognized as possibly the most destructive phenomenon in the modern-day electronics industry. ESD is ubiquitous in the handling, packaging and shipping of electronic devices and components and leads to extensive damage by causing excessive internal heating or dielectric breakdown when such devices lie in the discharge path. Since static electricity cannot be eliminated altogether, a variety of approaches have been developed for restricting the destructive effects of ESD.
For instance, static-sensitive components have been surrounded in various degrees by static-protective materials, depending on the properties of the material, so as to minimize charge generation and dissipate built-up charge by redistributing it on a surface, by providing a conductive path to ground, or by serving as a shield against external electrostatic fields. A popular approach in this regard has been the advantageous use of the "Faraday-cage" effect by means of a container (typically made of paper board) which is provided with a conductive coating (typically, a layer of carbon black) on its internal surface so that any electrostatic charge in the conductive layer is either bled off to ground or is forced to circulate harmlessly until the charge decays gradually to a negligible level. Such conductive containers, however, are incapable of providing ESD protection when the containers are open and are also susceptible to ESD due to triboelectric charge built up as a result of frictional action during insertion and removal of components; under these conditions, static-sensitive components remain subject to damage from static charges and fields.
Containers and envelopes formed of different types of laminated materials have also been used to control ESD problems. The laminated materials used with such static shielding products are produced by using heat or adhesive to laminate a thin sheet of flexible antistatic material to a conductive layer or grid of carbon or metal, which is then laminated to a support layer, typically polyester or paper backing. The laminated material works by conducting the static charge through a small segment of the volume of the surface material to the conductive layer to ground. Since the surface is an antistatic material, while the carbon or metal layer is electrically conductive, the bulk of the charge current passes through the conductive layer to ground. U.S. Pat. No. 4,699,830 to White and U.S. Pat. No. 4,738,882 to Rayford et al. are representative of such antistatic laminated sheet material which generally includes successive layers of antistatic material and conductive metal, with the laminated material being particularly adapted to forming protective containers or envelopes for electronic components.
It should be noted that the term "antistatic material", as used herein, is intended to include materials traditionally defined within the electronics industry (particularly, in manufacturing environments) as "antistatic" (typically having a surface resistivity of 10.sup.9 to 10.sup.14 ohms/square) as well as those defined as "static-dissipative" or "static shielding" (typically having a surface resistivity of 10.sup.5 to 10.sup.9 ohms/square).
Electronic components having leads are particularly susceptible to ESD problems because the conductive nature of the leads makes them the focal point for discharge of electrostatic charge, not only between the leads and surrounding static fields, but also from one lead to another. The prevalent practice in industry for protecting such leaded components is to supplement the use of static shielding containers for the same by shunting the components against static discharge. Shunting is accomplished by connecting all the component leads together through a common conductive path, thereby preventing a discharge through the component from one lead to another. A layer of conductive carbon foam (commonly known in the industry as "black" foam) is almost universally used for this purpose and leaded electronic components are supported on the foam by embedding the component leads into the foam.
While the use of conductive foam as a shunting support layer reduces the possibility of inter-lead electrostatic discharge through a component, it has several inherent disadvantages. Since the conductive foam must necessarily be highly conductive in order to provide effective shunting, it is possible to have electrostatic discharge even as contact is approached and made between component leads and the foam, if either of them happens to be statically charged. Another problem prevalent with the use of conductive foam is that the foam gets broken up into small conductive particles as the component leads penetrate the foam during the embedding process or, more frequently, when embedded component leads are extracted from the foam to release a component supported thereupon. Such conductive particles frequently get lodged between the leads resulting in damaging short-circuits in the components themselves or in the circuit assemblies on which the components are subsequently installed.
Accordingly, there exists a need for a static-protective material capable of avoiding the problems involved in protectively supporting static-sensitive electronic components having leads by using conductive shunt layers.